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Full text of "Handbook Of Chemical Engineering - I"

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466                              CHEMICAL ENGINEERING
The switch is then thrown to the right and since the factor of variable battery electromotive force or current is eliminated by this preliminary setting the scale of the galvanometer may be empirically graduated to read directly the temperature of the thermometer. Thus no further adjustments are necessary so long as this current remains steady, and any temperature is readily observed without the necessity for balancing required in the ordinary bridge. It is possible to have fcwo or more temperature scales on the galvanometer. When two scales are provided, as for example 0 to 300°C. and 300 to 700°C., one of the arms of the bridge is made adjustable to either of two resistances, one giving zero deflection for a thermometer temperature of 0°C. and the other zero deflection for a temperature of 300°C.
Summary on Resistance Thermometry.—Although the resistance thermometer is of the greatest importance in the laboratory for work of extremely high precision its importance for industrial processes at high temperatures is waning. The precision of which the instrument is capable requires many precautions easily overlooked. Bad contacts in the line, a< broken strand in a flexible cable, etc., may cause large errors. The thermometer is very fragile and its calibration may be altered by severe handling or by the slightest contamination or by volatilization of the platinum wire forming the coil. The range is confined usually below about 1,000°C., whereas rare-metal thermocouples may be used to 1,400°C.
In the early development of pyrometry before satisfactory galvanometers were devised for thermocouples, the resistance thermometer afforded about the only accurate means of temperature measurement, but with the highly sensitive and reliable thermocouple indicators and portable potentiometers and with the reproducible Le Chatelier couple now obtainable, there are few industrial processes carried out above 300°C. where a thermoelectric installation is not to be preferred. When very small temperature changes or intervals must be measured with high precision the resistance thermometer may be employed effectually. Also in certain processes where an integrated or average temperature over the entire bulb is required, the resistance thermometer is high desirable. A homogeneous thermocouple indicates the temperature of its hot junction while a resistance thermometer indicates the average temperature of the entire coil or bulb, which may be constructed as long as desired. There are no cold junction corrections for the resistance thermometer—a decided advantage over the thermocouple, but one which is offset by the disadvantages mentioned.
Recording Pyrometry.—The pyrometers which can be made to record automatically fall under the following classifications: (1) Gas, saturated vapor, and liquid thermometers; (2) resistance thermometers; (3) thermoelectric pyrometers; (4) radiation pyrometers.
Of these four types the thermoelectric pyrometer recorder has the greatest applicability, especially for the higher temperatures at which the first two named are not suitable. The constant-volume industrial gas thermometer is successful up to about 400°C. The resistance thermometer is capable of high accuracy up to 1,000°C. At such high temperatures, however, thermocouples are more serviceable since deterioration of the wire from continual heating does not so seriously alter the electromotive force developed by a couple as it does the resistance of a resistance thermometer. Radiation pyrometers are useful at the highest attainable temperatures, but processes in which extremely high temperatures are employed are not in general subjected to precise temperature control.
The most usual form of temperature record is that in which temperature appears as